This invention relates to ultrasonic transducers.
The conventional ultrasonic transducers generally have the piezoelectric elements sandwiched between two flanged metal blocks which are clamped to each other by fastening means such as bolts threaded into the flange portions of the respective blocks. With this arrangement, difficulties are encountered in that large amplitude flexural vibration is imparted to the flange portions during long ultrasonic operations, resulting in cracking or breakage of the sandwiched piezoelectric elements.
The conventional vibration amplitude magnifying type ultrasonic transducers usually employ an arrangement in which an ultrasonic transducer which has a half wavelength fundamental longitudinal resonance vibration system for converting electric oscillations into mechanical vibrations is coupled in series to an ultrasonic horn which has another half wavelength fundamental longitudinal resonance vibration system for magnifying the amplitude of the mechanical vibration, by suitable securing means such as soldering, bolting and the like. Such a vibration amplitude magnifying type ultrasonic transducer has a drawback in that it is large-sized and heavy because it consists of two fundamental longitudinal resonance vibration system coupled in series, viz., an ultrasonic transducer portion of half wavelength and an ultrasonic horn similarly of half wavelength, and thus necessarily has to have a length corresponding to one wavelength. With the vibration amplitude magnifying type ultrasonic transducer which has two fundamental resonance systems coupled in series, the intrinsic resonance frequencies of the two systems have to coincide perfectly with each other in order to generate ultrasonic vibrations effectively. In utilizing the ultrasonic vibrational energy in industrial applications, it is often the case that a machining tool or an attachment such as vibratory plate is fixed at the front end of the ultrasonic horn. In such a case, the intrinsic resonance frequency of the ultrasonic horn is influenced by the weight, shape and dimension of the attachment which is mounted at the front end of the horn as well as by the load which is externally imposed for doing some job. Therefore, the ultrasonic horns of the conventional vibration amplitude magnifying type transducers have to be designed and fabricated to have a resonance frequency which coincides with the resonance frequency of the ultrasonic transducer portion under actual working conditions. As a result, the designing and fabrication of the ultrasonic horns heretofore involved complicated calculations and experiments in determining the dimensions of the horns. Namely, enormous labor and experience have been required in designing and fabricating the vibration amplitude magnifying type transducers of conventional construction.
In an attempt to eliminate those drawbacks, the present inventors disclosed a vibration amplitude magnifying type ultrasonic transducer in their copending application (Japanese Patent Application No. 49-113147). As shown in FIG. 1(a), such ultrasonic transducer consists of a mechanical vibration magnifying block A with a flange A1 of large diameter located at a position distant from the mechanical vibration output end A2 by a length corresponding to one-quarter of the transmitting ultrasonic wavelength, for receiving a plural number of bolts E, a backing block B consisting of a cylindrical block of a predetermined length and having at its base end a circular flange B1 of predetermined wall thickness, a pair of piezoelectric elements C1 and C2 interposed between the aforementioned flanges, and bolts E and nuts F fastening the opposing flanges to each other through an annular support plate D which is in engagement with the flange A1 of the mechanical vibration magnifying block A. This transducer has the flange portions and the piezoelectric elements located in the vicinity of a point (at the node of longitudinal vibration mode) distant by a length corresponding to one-quarter of ultrasonic wavelength from the mechanical vibration output end A2 which is located at the antinode of the longitudinal vibration mode of the mechanical vibration magnifying block A, and has the other end B2 of the backing block provided at a point (at the antinode of longitudinal vibration mode), distant by a length corresponding to a half ultrasonic wavelength, thus acting as an ultrasonic transducer with a half wavelength fundamental longitudinal resonance vibration system as a whole and at the same time functioning as an ultrasonic horn of a half wavelength fundamental resonance vibration system for the magnification of the amplitude. Therefore, the transducer is extremely compact in construction and light-weight and can find various applications in those fields where there are severe spatial restrictions.
However, the transducer of the above construction still can cause cracking to the piezoelectric element such as PZT which is abutted against the flange portion of the mechanical vibration magnifying block, due to the flexural vibration of the block which might be imparted thereto when the transducer is vibrated at large amplitude continuously over an extremely long time period, with resultant transitional variations in electric impedance and resonance frequency of the transducer. Similarly to the conventional devices, the above-described transducer also has a problem in that, when the transducer is fixedly supported on an external structure through an annular support plate D which is provided at the node of the longitudinal vibration, the fixed support of the vibratory element entails energy losses and deteriorations in the operating characteristics of the transducer.
The results of our study on the causes of the above problems are now explained with reference to FIG. 1(b). The transducer T has the node of its longitudinal vibration in the vicinity of the center point G of annular flat surface A11 of the mechanical vibration magnifying block A, the respective parts along the axis of the transducer resonating in the mode with longitudinal vibrational displacements as shown in the graph of FIG. 1.
This vibration causes longitudinal vibration L having vibrational displacements parallel to the axis of the transducer, but, concurrently with axial vibrational distortions, there also occur radial vibrational distortions in an amount according to Poisson's ratio. As a result, the transducer also has radial vibration R, expanding and contracting in the radial directions though in a slight degree. The radial vibrational displacement is largest at the node of vibration where the stress of the longitudinal vibration is maximum or the displacement of the longitudinal vibration is zero as shown by the dotted lines in the graph of FIG. 1. This radial vibrational displacement induces and causes flexural vibration K to the flange portion A1 of the mechanical vibration magnifying portion A, imparting curved vibrational displacements to the flat end surface A3 of the flange portion and imposing bending load repeatedly on the piezoelectric element. The piezoelectric elements are therefore susceptible to cracking damages especially in a long drive in large amplitude.
The above transducer has the flange A1 of the mechanical vibration magnifying portion formed in a large diameter to receive a number of bolts E which are employed as clamping means and constructed to permit of suitable elastic deformation upon bending deformation of the flange, resulting in inducement of undesirable flexural vibrations to the flange portion as described hereinbefore.
In addition, the conventional transducer has the annular support plate D arranged simply to provide uniform and resilient support for the flange A1, failing to restrict or suppress the flexural vibrations of the flange and to let the entire area of the annular flat surface A11 of the mechanical vibration magnifying block A act perfectly as a node of the longitudinal vibration. Therefore, the annular support plate D is allowed to vibrate, though in a slight degree, concurrently with the vibration of the transducer, influencing the resonance characteristics of the transducer. This causes losses of vibrating energy and deterioration of operating characteristics in the case where the transducer is fixedly supported on an external structure through the annular support plate D.